Semiconductor devices have dramatically decreased in size in the last few decades. Modern devices include features that are 350 nanometers, 90 nanometers, and 65 nanometers in size and often even smaller. As device and feature sizes continue to shrink, processing methods need to be improved.
Silicon nitride films have been widely adopted by the semiconductor industry. For example, silicon nitride may be formed as temporary spacers in metal oxide semiconductor (MOS) devices, such as metal oxide semiconductor field effect transistors (MOSFETs). Specifically, silicon nitride spacers may be used during fabrication of lightly doped drain (LDD) regions to facilitate different levels of doping for the drain/source regions as well as for the LDD regions. The LDD region can be controlled by the lateral spacer dimension. Removal of silicon nitride spacers can easily damage adjacent structures, such as metal silicide layers, gates, and underlying silicon substrates.
The silicon nitride spacers are conventionally removed using a boiling or near boiling phosphoric acid solution. The concentration of phosphoric acid in the solution is kept high (e.g., above 85%) in order to achieve higher operating temperatures (e.g., 160° C. and above) and faster etching rates. Other approaches to remove silicon nitride structure involve dry etch. However, these processes often lead to erosion of metal silicides (e.g., nickel silicide) of the MOS structures.